Sphenoidal electrodes, which were introduced in the late 1940s and early 1950s, are reported to be superior to nasopharyngeal electrodes and ear electrodes for detecting mesial temporal lobe epileptiform discharges. See "Mesial Temporal Spikes: A Simultaneous Comparison of Sphenoidal, Nasopharyngeal, and Ear Electrodes," by Sperling et al., in Epilepsia, Vol. 27, No. 1 (1986).
A sphenoidal electrode is a wire which is inserted by a physician into the jaw muscle and nearby tissue to a position near the sphenoid bone where it remains during a test period to sense epileptiform discharges. More specifically, the site of insertion is approximately 3-4 mm below the zygoma and 2-3 cm in front of the tragus.
In one insertion technique, the sphenoidal wire electrode is inserted by means of a hollow needle which contains a straight section of such wire terminating in a straight distal end. During insertion, the needle and wire are advanced medially until the tip of the needle strikes bone. The needle is then withdrawn along the wire leaving the wire inserted for the duration of the test period. During the test period, which may extend for as long as five days or more, the opposite end of the sphenoidal electrode wire is electrically connected to an EEG jack box, usually via another wire. After the test period, the wire electrode is removed by pulling it out.
While sphenoidal wire electrodes are preferred for detecting mesial temporal lobe epileptiform discharges, significant problems have been experienced in proper insertion of such electrodes. Inserting the electrode wire such that its distal end reliably arrives and stays at the intended location for optimal discharge detection may be difficult for various reasons.
There is a tendency during insertion of the needle and contained wire for the wire to be pushed along the needle, farther into the needle, away from the needle tip. This may be caused by tissue which impacts the wire during the insertion motion.
Improper location of the distal end of the sphenoidal electrode wire may also be caused and/or exacerbated by a tendency for the wire to retract as the insertion needle is withdrawn along the wire prior to beginning of the test period. The relative movement of the needle along the wire tends to frictionally pull it in a withdrawal direction.
Also, during the test period itself, when the wire electrode is intended to remain in place, muscular motions of the patient can tend to relocate to some extent the position of the distal end of the sphenoidal electrode. This can cause detection of discharges to vary improperly over the test period.
Whatever the tendencies or reasons for improper location of the distal end of the sphenoidal wire electrode, the fact of and the extent of any such improper location are not readily discerned. Thus, when to take corrective measures during insertion procedures is often unknown.